ARS | Publication request: The status of Bombus occidentalis and B. moderatus in Alaska with special focus on Nosema bombi incidence

Submitted to: Northwest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 22, 2012
Publication Date: N/A

Interpretive Summary: Of the 50 species of bumble bees found in North America, it is known that five species are declining in range and abundance. Four of these species are closely related and it is suspected that this relationship may be tied to the declines. We investigated the distribution and abundance of one of the known declining species and a fifth related species that does not occur in the contiguous United States. Because the bumble bees of Alaska are more isolated, we hypothesized that they may not be subject to the same pressures causing decline as we found in previous studies in the lower 48 states. Additionally, we survey for a fungal pathogen that is correlated to decline by dissecting the bees and examining gut contents under a microscope. We found that both of these bee species are widespread and common in Alaska. One species, B. occidentalis, is the most common species, yet has the highest infection rate of all surveyed bees. We conclude that bumble bee populations in Alaska seem healthy, but should be monitored for changes in the future.

Technical Abstract: Technical Abstract: Four North American bumble bee species in the subgenus Bombus sensu stricto, including Bombus occidentalis (Hymenoptera: Apidae), are experiencing dramatic declines in population abundance, range and genetic diversity. The prevailing hypothesis concerning their decline is the ‘spillover’ of the intracellular microsporidian pathogen, Nosema bombi (Microsporida), and other pathogen species from commercially reared bumble bees to wild populations. While there is a great deal of effort to determine the health of bumble bees in the contiguous U.S.A. and southern Canada, virtually nothing is known about more isolated northern bumble bee populations where absence of commercial pollinations services reduces the likelihood of pathogen spillover. This study presents the distribution, relative abundance, and prevalence of N. bombi for B. occidentalis and B. moderatus, two co-occurring species in the subgenus Bombus sensu stricto in Alaska. Bombus occidentalis and B. moderatus account for 36% and 11% of the bumble bee fauna surveyed, respectively. Bombus occidentalis was the most abundant species collected in the survey and prevalence of N. bombi infections (44% infected) was also highest. The proportion of infected B. moderatus did not differ significantly from other co-occurring bumble bees. Despite the presence of N. bombi infections, both Bombus s. str. species were commonly detected in Alaska with our survey method. Alaskan bumble bee populations may thus provide important insights on the role of pathogens, particularly N. bombi, in bumble bee decline in the contiguous U.S.A.